\documentclass{article}
\usepackage{booktabs,makecell,multirow,tabularx}
\renewcommand\theadfont{\bfseries\normalsize}
\newcolumntype{C}{>{\centering\arraybackslash}X}

\usepackage{geometry}

\begin{document}
\begin{table}[ht]
	\centering
	\begin{tabularx}{\textwidth}{c L c L c L c }
		\toprule
		\multirow{2}{*}{\thead{Atlas                                          \\ Resolution}}
		                   & \multicolumn{2}{c}{\thead{Variant-1}}
		                   & \multicolumn{2}{c}{\thead{Variant-2}}
		                   & \multicolumn{2}{c}{\thead{Variant-3}}            \\
		\cmidrule(lr){2-3}\cmidrule(lr){4-5}\cmidrule(lr){6-7}
		                   & \thead[b]{Average                                \\ Render\\ Time (ms)}  &   \thead[b]{IFPS}
		                   & \thead[b]{Average                                \\ Render\\ Time (ms)}  &   \thead[b]{IFPS}
		                   & \thead[b]{Average                                \\ Render\\ Time (ms)}  &   \thead[b]{IFPS}                \\
		\midrule
		512$\times$512     &                   &                   &  &  &  & \\
		1024$\times$1024   &                   &                   &  &  &  & \\
		\addlinespace[2pt]
		18432$\times$18432 &                   &                   &  &  &  & \\
		19456$\times$19456 &                   &                   &  &  &  & \\
		\bottomrule
	\end{tabularx}
	\caption{List of sizes for various resolution of Dataset-I, compressed variant Dataset-II and different slices variant Dataset-II}
	\label{tab:1}
\end{table}
\end{document}

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